80 SPECIAL SENSES. 



tory passage, as fishes, or without semicircular canals, as crabs, 

 perceive sounds in a very imperfect manner. 



3. OF SMELL. 



162. SMELL is the faculty of perceiving odours, and is 

 a highly important sense in many animals. Like sight and 

 hearing, smell depends upon special nerves, the olfactory, which 

 form the first pair of cerebral nerves (fig. 20, i\ and which, 

 in the embryo, are direct prolongations of the brain. 



163. The organ of smell is the NOSE. Throughout the 

 series of vertebrata it makes a part of the face, and in man, by 

 reason of its prominent form, it becomes one of the dominant 

 traits of his countenance ; in other mammals, the nose, by de- 

 grees, loses this prominency, and the nostrils no longer open 

 downwards, but forwards. In birds, the position of the nos- 

 trils is a little different ; they open farther back, and higher 

 up, at the origin of the beak. 



164. The nostrils are usually two in number some fishes 

 have four. They are similar openings, separated by a partition 

 upon the middle line of the body. In man and the mammals, 

 the outer walls of the nose are composed of cartilage ; but 

 internally, the nostrils communicate with cavities situated in 

 the bones of the face and forehead. These cavities are lined 

 by a thick membrane, the pituitary, on which are expanded 

 the olfactory nerves, [and some filaments of the fifth pair.] 



165. The process of smelling is as follows. Odours are 

 particles of extreme delicacy, which escape from very many 

 bodies, and are diffused through the air. These particles make 

 an impression on the nerves of smell, which transmit the im- 

 pressions to the brain. To facilitate the perception of odours, 

 the nostrils are placed in the course of the respiratory passages, 

 so that many of the odours diffused in the air, which are in- 

 spired, pass over the pituitary membrane. 



166. The acuteness of the sense of smell depends on the 

 extent to which that membrane is developed. Man is not so well 

 endowed in this respect as many mammals, which have the in- 

 ternal surface of the nostrils extremely complicated. Such is 

 especially the case among the carnivora. 



167. The sense of smell in reptiles is less delicate than 

 in mammals; their pituitary membrane being less developed. 



